The Telegraph reports that Philip's death certificate was certified by Sir Huw Thomas, Head of the Medical Household and Physician to the Queen.
The determination of "old age" is reportedly an accepted cause of death in England for people over the age of 80 who have also received medical care from the same physician over a long period of time, with no other significant factors leading to their demise.
According to the Telegraph, it indicates that there was no specifically identifiable cause.
Philip's death came after several health scares in recent years. Most recently, he was hospitalized in February and taken to the King Edward VII Hospital in London. In a statement, Buckingham Palace noted that it was "a precautionary measure" on the advice of his doctor after not feeling well.
Prince Phillip officially retired from public duties in 2018. Ongoing struggles with his health forced him to cancel multiple public appearances including the Royal Maundy Service at St. George’s Chapel in 2018, which he pulled out of after experiencing trouble with his hip. He also skipped an Easter Sunday services at the chapel, although the Queen, Prince William and Kate Middleton were in attendance. Prince Phillip underwent hip surgery in April 2018.